Multiple Acquisition of Methanogenic Archaeal Symbionts by Anaerobic Ciliates

The establishment of an endosymbiotic relationship typically seems to be driven through complementation of the host's limited metabolic capabilities by the biochemical versatility of the endosymbiont. The most significant examples of endosymbiosis are represented by the endosymbiotic acquisition of plastids and mitochondria, introducing photosynthesis and respiration to eukaryotes. However, there are numerous other endosymbioses that evolved more recently and repeatedly across the tree of life. Recent advances in genome sequencing technology have led to a better understanding of the physiological basis of many endosymbiotic associations. This review focuses on endosymbionts in protists (unicellular eukaryotes). Selected examples illustrate the incorporation of various new biochemical functions, such as photosynthesis, nitrogen fixation and recycling, and methanogenesis, into protist hosts by prokaryotic endosymbionts. Furthermore, photosynthetic eukaryotic endosymbionts display a great diversity of modes of integration into different protist hosts.In conclusion, endosymbiosis seems to represent a general evolutionary strategy of protists to acquire novel biochemical functions and is thus an important source of genetic innovation.

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“…Metopus contortus, Trimyema sp. and Cyclidium porcatum (Fenchel & Finlay 1991a;van Hoek et al 2000). These ciliates contain hydrogenosomes, i.e.…”

Section: Prokaryotic Endosymbionts In Protists (A) Photosynthetic Endmentioning

confidence: 99%

Endosymbiotic associations within protists

Nowack

Melkonian

2010

Phil. Trans. R. Soc. B

208

168

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“…The hydrogenosomes of ciliates have been viewed as problematic because they are found among clades of anaerobic ciliates that are near relatives of aerobic clades with mitochondria (54,62,183,184). Thus, the phylogenetic reconstructions suggest that the hydrogenosomes in these clades of ciliates must have arisen independently several times.…”

Section: Hydrogen Exporter-earlymentioning

confidence: 99%

Origin and Evolution of the Mitochondrial Proteome

Kurland

Andersson

2000

Microbiol Mol Biol Rev

362

272

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SUMMARY The endosymbiotic theory for the origin of mitochondria requires substantial modification. The three identifiable ancestral sources to the proteome of mitochondria are proteins descended from the ancestral α-proteobacteria symbiont, proteins with no homology to bacterial orthologs, and diverse proteins with bacterial affinities not derived from α-proteobacteria. Random mutations in the form of deletions large and small seem to have eliminated nonessential genes from the endosymbiont-mitochondrial genome lineages. This process, together with the transfer of genes from the endosymbiont-mitochondrial genome to nuclei, has led to a marked reduction in the size of mitochondrial genomes. All proteins of bacterial descent that are encoded by nuclear genes were probably transferred by the same mechanism, involving the disintegration of mitochondria or bacteria by the intracellular membranous vacuoles of cells to release nucleic acid fragments that transform the nuclear genome. This ongoing process has intermittently introduced bacterial genes to nuclear genomes. The genomes of the last common ancestor of all organisms, in particular of mitochondria, encoded cytochrome oxidase homologues. There are no phylogenetic indications either in the mitochondrial proteome or in the nuclear genomes that the initial or subsequent function of the ancestor to the mitochondria was anaerobic. In contrast, there are indications that relatively advanced eukaryotes adapted to anaerobiosis by dismantling their mitochondria and refitting them as hydrogenosomes. Accordingly, a continuous history of aerobic respiration seems to have been the fate of most mitochondrial lineages. The initial phases of this history may have involved aerobic respiration by the symbiont functioning as a scavenger of toxic oxygen. The transition to mitochondria capable of active ATP export to the host cell seems to have required recruitment of eukaryotic ATP transport proteins from the nucleus. The identity of the ancestral host of the α-proteobacterial endosymbiont is unclear, but there is no indication that it was an autotroph. There are no indications of a specific α-proteobacterial origin to genes for glycolysis. In the absence of data to the contrary, it is assumed that the ancestral host cell was a heterotroph.

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“…All other species in 2 and 3 are free-living (c.f. van Hoek et al, 1998van Hoek et al, , 2000b, all species displayed in box 1 are living in the rumen of ruminants. van Hoek et al, 1998Hoek et al, , 1999Hoek et al, , 2000b.…”

Section: Nyctotherus Ovalismentioning

confidence: 99%